I attended a seminar once where after the second day, there was so much accumulated sweat on the mats that they were as slippery as ICE! Then I discovered that no one had mopped them after the FIRST day.

That was nasty enough. After a few days back home, I noticed a small red place on the inside of my knee. Itched like hell and I began treating it immediately. Fortunately it was a very small spot but I wouldn't allow myself back on the mats until it was gone.

Naturally you have to stay on top of your OWN hygene. It's that of OTHERS that you can't control. Wearing gi pants and a long sleeve shirt/gi top is one way of reducing your chances of getting it. I don't tend to follow my own advise often in that regard...